The invention relates to an arrangement and a method for controlling the amount of a reducing agent which is supplied to an exhaust line of a combustion engine.
One way of reducing emissions of nitrogen oxides from diesel engines is to use a technique called SCR (selective catalytic reduction). This involves a reducing agent in the form of a urea solution being supplied in a specific dose to the exhaust gases in the exhaust line of a diesel engine. When urea solution is sprayed into the exhaust line, the resulting finely divided solution becomes vaporised by contact with the hot exhaust gases, and ammonia is formed. The mixture of ammonia and exhaust gases is then led through an SCR catalyst in which the nitrogen in the nitrogen oxides in the exhaust gases reacts with the nitrogen in the ammonia to form nitrogen gas. The oxygen in the nitrogen oxides reacts with the hydrogen in the ammonia to form water. The nitrogen oxides in the exhaust gases are thus reduced in the catalyst to nitrogen gas and water vapour. With correct dosage of urea, the diesel engine's emissions of nitrogen oxides may be greatly reduced.
The dosage of the urea solution is usually based on measurements of the amount of nitrogen oxides NO before the SCR catalyst, but the measuring accuracy of a nitrogen oxide sensor is not particularly good. A nitrogen oxide content value measured by a nitrogen oxide sensor may differ from the actual value by about 10%. Moreover, the measuring accuracy of nitrogen oxide sensors worsens with age.
In particular, when too large amounts of urea solution are added, ammonia passes through the SCR catalyst without reacting with nitrogen oxides in the exhaust gases. To eliminate this ammonia, an ammonia catalyst is usually provided downstream of the SCR catalyst. The function of the ammonia catalyst is to convert ammonia to nitrogen gas. A problem with ammonia catalysts is that in addition to nitrogen gas they also form nitrous oxide (N2O). Nitrous oxide is a greenhouse gas about 300 times stronger than carbon dioxide. It is therefore very important for discharges of nitrous oxide to be reduced. Too high dosage of urea solution also increases the risk that urea lumps might form in the exhaust line.